This is a report. In teamwork mode, if the layer object set is not reserved, a layer localy created and not sent could not be modified by the ACAPI_Attribute_Modify. In such case, ACAPI_Attribute_Modify returns 0(=NoError). But though I immediately got the layer parameters by ACAPI_Attribute_Get, nothing had changed in them. Instead of ACAPI_Attribute_Modify, I was able to modify the layer by attribute delete/create API functions. (Actually, it is need some additional code to avoid deleting the element associated with the layer we're modifying.) The flags field in this layer had the APIAttr_IsClientCreated flag was ON. In the API_Attr_Head document, description of the APIAttr_IsClientCreated says that "such an attribute can still be modified or deleted in teamwork mode". But it seems has some restrictions.